1. Field of the Invention
The present invention relates to connectors used to erect three-dimensional arrays of polyhedral elements made from prismatic or nonprismatic structural members. Specifically the invention relates to a pinned connection between the members and socketed central nodes to facilitate rapid assembly and behavior which can be predicted, especially when the members are subjected to large compressive loads.
2. Description of the Background Art
Three-dimensional arrays of polyhedral elements can be formed into flat or curved lattices which may be enveloped by a cover shell which transmits external pressures to the lattice. These structures are highly suitable for the coverage of large areas such as sports arenas etc., for the enclosure of large volumes such as storage tanks, and for use in different applications in outer space.
The fundamental behavior of lattice structures differ depending on the type of connectors used to attach the member ends to the nodes. In a structural sense, connector types vary from being pinned to fully rigid. Pinned connectors offer no bending resistance and rotational continuity between the members common to a central node. Fully rigid connectors maintain complete continuity and transfer bending moments between the members and the nodes. Differences in behavior arise because of the stability characteristics of admissible deformation modes in postbuckling and the influence of imperfections. Imperfections reduce the load-carrying-capacity of real structures from theoretical predictions by amounts which are strongly dependent on the stability of the structure under high loads. In a lattice with rigid connections, both stable and unstable postbuckling deformation modes exist. Since there is no reason for the lattice to select a high energy level over a lower level, the lattice will deform in the unstable mode, resulting in a reduced load carrying capacity from that theoretically predicted. The actual peak load may be as low as thirty percent of the theoretical value. If the mode is highly unstable, the structure will collapse in a violent manner.
In a pinned lattice, the postbuckling deformation modes may be stable or unstable. In general, these modes are not as unstable as those of rigidly jointed lattices and load-carrying-capacities are not as drastically reduced from theoretical values. If highly unstable modes exist, they may be mitigated by stiffening certain members in the lattice allowing the postbuckling modes to occur under relatively stable conditions, and at the same time inc ease the load-carrying-capacity.
Improved stability and increased strength require the accurate prediction of structural response. Currently a large gap exists between the theoretical and the actual response of lattice structures due to the dominating effect of the joint type. In practice, joints are neither rigid nor pinned but are semirigid. The added complexities involved in both the mathematical modeling of semirigid joints and their unknown behavior under different combinations of forces are not conducive for improving the stability and the load-carrying-capacity.
Connectors used for assembling lattices must be designed to enable a member to be inserted between two fixed nodes. The connectors must also permit a minor shortening or lengthening of the member. An additional advantage exists if the connector permits a minor deviation in the directional axis of the member from its predetermined geometry.
Several systems for constructing lattice structures used for covering large areas etc. exist. Below is a description of these systems and details of the connectors used to assemble the lattice. Further details are also given in Exhibit A.
The first of the modern space structures was the MERO system which was used extensively in 1957 at the International Building Exhibition in Berlin. This system uses a threaded bolt to attach tubular members fitted with cone-shaped endpieces to spherical socketed nodes. The attachment is made by turning a slotted nut which engages a pin fixed to the bolt. As the nut is turned, the bolt emerges from the nut and engages the threads in the socket of the spherical node. The MERO system makes it possible to insert a member between any two fixed nodes. This connection scheme is considered semirigid and its behavior is subjected to the unstable postbuckling deformation modes associated with lattices employing rigid connections. Also, this system requires exact alignment of the member axis with the socketed nodes necessitating extreme control during manufacture and assembly.
The Zublin Space Truss System has been on the market in Germany since 1977. This system consists of tubular members with tapered ends which are connected to spherical nodes by means of special bolts. The connector bolt has two threads of different pitch. On the node side, there is a standard pitch whereas on the member side there is a fine pitch. Between the two threads is either a hexagonal portion on which a wrench collar can slide or a threadless portion with two opposing studs on which a circular collar with internal grooves can slide. The collar serves as a spacer between the member and the node and is required for turning the bolt by means of a wrench. Before turning, there is play of several threads between the collar and the node. With each turn, the bolt is moved closer to the node by the difference between the two pitches, thus diminishing the play until the connection is tight. This connection is also semirigid because of the significant moment transferred by the bolt and collar to the nodes. Careful alignment is also required of the member axis and the axis of the sockets in the node.
Tomoegumi Iron Works, Ltd., of Japan developed the TOMOE UNITRUSS system. This system is composed of spherical socketed nodes, high strength connectors and tubular members with nose-cone endpieces. The connectors used for the attachment of the endpieces to the nodes consist of threaded portions on each end separated by a hexagonal segment to accommodate a hexagonal wrench. As the connector is turned it unscrews from the nose-cone and engages the threaded sockets in the nodes. Right-hand and left-handed screws on the member end of the connectors allow the distance between two nodes to be adjusted by rotating the structural members. On completion of assembly, set screws in the nose-cones are tightened to prevent the connections from becoming loose. This assembled structure is also considered semirigid due to the moment transferred by the bolts. In addition, careful alignment is required during assembly.
The NS Space Truss System developed by Nippon Steel Corporation of Japan uses spherical nodes, pipes with end cones and threaded bolt connectors. This system uses hollow spheres sufficiently large such that the bolts are inserted into the end cones from inside the sphere. Openings exist at the poles of the spheres to allow the bolts to be inserted and tightened with a rachet socket wrench. Washers are used between the nodes and the pipe members where the node side of the washer is concave to match the contours of the sphere. The washer bolt combination transfers significant moment to make this a semirigid connection. Another complexity is introduced via the usage of large hollow spheres which may undergo deformations larger than the axial deformations of the members. It appears that exact alignment is crucial for assembly of the NS System.
Other systems for constructing space structures also exist which do not rely on attaching members to nodal spheres. These employ a variety of hardware. Typical of these is the Temcor System in which aluminum wide flange sections are bolted to circular gusset plates. Such systems are limited to certain geometrical shapes and defy prediction because of mathematical complexities involved in modeling the joints.
A connection system for assembling space structures does not exist in which the connectors may be treated as pinned or quasipinned. Pinned connectors allow for the exploitation of the high load-carrying-capacity of space structures while at the same time make space structure lattices safer by improving the stability characteristics at high loads. Also, there is a need for a connector which facilitates rapid construction by permitting minor deviations in both axis alignment and member length.